The demand for portable consumer products with rechargeable batteries is extremely high and the marketplace for these types of products is highly competitive. Due to competitive pressure, manufacturers of portable electronic devices, including manufacturers of power supplies and chargers, are constantly looking for high quality, low cost designs. In some cases, product performance is sacrificed as cost reduction pressure is an overriding constraint.
Many low cost chargers are of the "ON/OFF" variety. They are so named because the output power is either fully on or fully off. ON/OFF chargers generally are capable of producing a specified output voltage with a current limit. In other words, the charger delivers a pre-determined voltage while limiting the maximum available current, and thus the maximum power that can be delivered.
Even less expensive chargers use systems where the power that can be delivered is limited only by the charger's source impedance. Here the voltage decreases as current increases, in a linear fashion. The output characteristics of such chargers can be seen in FIG. 1. These types of supplies, ON/OFF current limited and ON/OFF impedance limited, are very popular due to their low cost.
Such low cost designs are generally adequate in performance for nickel based battery systems, including nickel-cadmium and nickel-metal hydride. Nickel based batteries can be successfully charged over a wide range of charging currents while retaining much of the energy delivered by the charger as stored cell energy. It is typical for a battery cell to retain 80 to 90% of the delivered energy when using either a ON/OFF current limited or an ON/OFF impedance limited charger.
Much more efficient charging, however, can be obtained when a "top-off charge" feature is added. Top-off charge refers to the process of supplying the battery cell with small amounts of energy when a termination voltage is reached. In ON/OFF chargers, this is accomplished by rapidly turning the charger on and off, or pulsing the charger in the on mode and thereby delivering bursts of energy to the battery. Nearly 100% charge capability is obtainable in nickel systems using this method.
The problem associated with ON/OFF designs is that they are not compatible with lithium based batteries like lithium-ion and lithium polymer. Lithium based batteries have traditionally required constant current--constant voltage chargers. Constant current--constant voltage chargers work as follows: when a battery is discharged, the voltage dips to a low value, 3 volts for example. When the discharged battery is connected to a constant current--constant voltage charger, the charger initially charges at a constant current, 1 amp for example, until the voltage of the battery rises to a preset termination point, 4 volts for example. The charger then reduces the current and continues to charge the battery at a constant voltage which is the termination voltage. Hence the constant current--constant voltage name. ON/OFF chargers are not compatible with lithium batteries because they are not able to reduce the current at the constant termination voltage. It is therefore difficult to design multiple chemistry, i.e. lithium compatible and nickel compatible, chargers with the ON/OFF topology.
A compromise charging system for lithium batteries can be found using an ON/OFF charger. The charger charges in the ON/OFF mode until the lithium termination voltage is reached. Once the termination is reached, the ON/OFF switch electrically disconnects the charger from the battery. The drawback to this method is that lithium cells only get charged to about 80% of their capacity.
There is therefore a need for an inexpensive charger that can rapidly charge both nickel based and lithium based batteries.